Fast programming memory device

ABSTRACT

In an embodiment of a memory device including a matrix of memory cells wherein the memory cells are arranged in a plurality of memory cells strings each one including at least two serially-connected memory cells, groups of at least two memory cells strings being connected to a respective bit line, and wherein said memory cells are adapted to be programmed into at least a first programming state and a second programming state, a method of storing data comprising exploiting a single memory cell for each of the memory cells string for writing the data, wherein said exploiting includes bringing the single memory cell to the second programming state, the remaining memory cells of the string being left in the first programming state.

PRIORITY CLAIM

This application claims priority from Italian patent application No.MI2007A001012, filed May 18, 2007, which is incorporated herein byreference.

TECHNICAL FIELD

An embodiment of the present invention relates to the electronics field.More specifically, an embodiment of the present invention relates toelectronic systems employing memory devices.

BACKGROUND

In a number of electronic systems, the memory devices are used to storerecovery data necessary for restoring a working environment in case thepower supply goes down.

For example, when the power supply is cut off abruptly in a computer(due to an interruption of the electric power or any mistake of theuser), recovery data have to be stored in the memory devices just beforethe power down, so that when the power supply is applied again to thecomputer, the working environment related to the former state precedingthe power down is restored using the recovery data. Such operation isalso called a “hibernation operation”.

Typically, non-volatile memory devices (such as conventional flash orEEPROM memories) are employed for storing the recovery data during thehibernation operation. The data are safely saved by programming thenon-volatile memory devices; and the programmed data are preserved evenwhen the memory device power supply is off.

Unfortunately, the program operation of flash or EEPROM memories mayrequire a significantly long time, so that these memories may beunsuitable for applications where the hibernation operation has to beperformed in a very short time (ranging for example from 50 μs to 500μs).

In more detail, the flash memory devices are electrically-programmable,non-volatile memories, with memory cells consisting of a floating-gateMOS transistor, which stores a logic value defined by its thresholdvoltage (which depends on the electric charge stored on the floatinggate). In particular, during the programming operation of thefloating-gate MOS transistor electrons are injected, for example bymeans of the known Fowler-Nordheim tunneling mechanism, into thefloating gate; the retention of such electrons by the floating gateguarantees the memory device non-volatility. Vice versa, during theerasing operation, the floating gate is substantially emptied of theelectrons, which were injected during the programming.

In a flash memory, every single memory cell may be programmedindividually, but memory cells cannot be individually erased: a largenumber of memory cells, forming a so-called “memory sector”, are erasedat the same time.

In a flash memory device with NAND architecture, which ensures a highdegree of compactness, the memory cells are grouped in strings, each oneconsisting of a group of memory cells that are connected in seriesbetween a drain select transistor, connected to a bit line, and a sourceselect transistor, connected to a reference voltage distribution linedistributing a reference voltage (such as a ground voltage); groups ofsaid strings of memory cells are connected to a same bit line, and thusthey are connected in parallel to each other.

Memory cells are programmed by repeatedly applying thereto programmingvoltage pulses.

During a reading operation, the select lines of the string including theselected memory cell to be read are brought to the supply voltage. Theword line of the selected memory cell is brought to a read voltage whichis intermediate between the threshold voltage range corresponding to the“1” state and the threshold voltage range corresponding to the “0”state; for example, the read voltage may be equal to the referencevoltage (0V). The other word lines, to which the other memory cells ofthe string belong, are brought to a passing voltage Vpass having a value(for example, 4.5V) sufficiently high to ensure that the memory cellsconnected thereto are rendered conductive irrespective of theirthreshold voltage, i.e. irrespective of the fact that they areprogrammed or not.

Thus, by using the reference voltage as a reading voltage value, theselected memory cell of the string will be conductive if its thresholdvoltage is lower than the gate-source voltage applied thereto, and notconductive otherwise; this allows discriminating the data stored in thememory cells.

In order to ensure that all the other memory cells of string, other thanthe selected memory cell, are conductive, the passing voltage Vpass ischosen sufficiently higher than the expected maximum threshold voltageof the memory cells of the string. If even one memory cell happens tohave a threshold voltage higher than the expected maximum thresholdvoltage, the latter may remain non-conductive when the passing voltageVpass is applied thereto, and this causes errors of the read logicvalue.

Such problem may be magnified by the fact that during each readingoperation, spurious hot carrier generation occurs that cause softprogramming of the unselected memory cells. In such case, the thresholdvoltage of some memory cells may become higher that the expected maximumthreshold voltage, so that even applying thereto the passing voltageVpass those memory cells do not turn on, causing reading errors.

For limiting the occurrence of these problems, the width of thestatistical distribution of the threshold voltages of the programmedcells should be tightly controlled, so that the threshold voltage variesin a quite narrow range of values (for example, 3V). In order to ensurethat the width of the memory cells threshold voltages distributionremains within the desired range, several verify operations are to beperformed, after each programming pulse. The need to apply severalprogramming pulses and to perform several program verify lengthens thetime required for programming the memory.

SUMMARY

According to an embodiment of the present invention, a reduced number ofmemory cells are used, for example in an hibernation operation, forstoring the recovering data to be exploited when the power supplyreturns for recovering the system operating state before the powersupply powers down.

An embodiment of the present invention proposes a method of storing datain a memory device including a matrix of memory cells having a NANDarchitecture. The memory cells are arranged in a plurality of memorycells strings each one including at least two serially-connected memorycells, groups of memory cells strings are connected to respective bitlines; said memory cells are adapted to be programmed into at least afirst programming state and a second programming state. A method ofstoring data comprises: exploiting only one memory cell for each of thememory cells strings connected to a same bit line for writing the data,wherein said exploiting includes bringing the memory cell to the secondprogramming state, the remaining memory cells of the string being leftin the first programming state.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages will be made apparent by thefollowing detailed description of one or more embodiments of theinvention, provided merely by way of non-limitative example, descriptionthat will be conducted making reference to the attached drawings,wherein:

FIG. 1 schematically shows a block diagram of a semiconductor memorydevice according to an embodiment of the present invention;

FIG. 2 schematically shows a block diagram of a semiconductor memorydevice according to another embodiment of the present invention;

FIG. 3 shows, by way of a schematic flow-chart, a programming procedureof the memory device of FIG. 2 according to an embodiment of the presentinvention; and

FIG. 4 shows an exemplary electronic system wherein the semiconductordevice according to an embodiment of the present invention isadvantageously employed.

DETAILED DESCRIPTION

Throughout the following description, identical or similar elements inthe drawings are denoted by same reference numerals.

With reference to the drawings, in FIG. 1 a non-volatile semiconductormemory device 100 is illustrated, particularly an electricallyprogrammable, non-volatile semiconductor memory, for example a flashmemory.

The semiconductor memory device 100 is integrated in a chip ofsemiconductor material, and includes a plurality of memory sectors 105.The generic memory sector 105 is erasable individually, independentlyfrom the other memory sectors; each memory sector 105 comprises aplurality of memory cells 110, arranged in a plurality of rows and aplurality of columns, and which when the memory sector 105 is erased areall erased at the same time.

Typically, each memory cell 110 consists of an n-channel MOS transistorhaving a charge-storage element, such as a conductive floating gateadapted to be charged by electrons injected into it. In an erasedcondition (associated by convention with the logic value “1”), thememory cell 110 has a low (negative compared to a reference voltage,like the ground, taken as the zero voltage) threshold voltage valuewhereas in the programmed condition (associated by convention with thelogic value “0”), the memory cell 110 has a high (positive) thresholdvoltage value. For reading the content of a selected memory cell 110,the cell is biased so as to be conductive if it is erased, ornon-conductive if it is programmed.

The semiconductor memory device 100 has NAND architecture: in eachsector 105, groups of, for example, eight or sixteen, memory cells 110are connected in series to each other to form a respective memory cellsstring, and different memory cells strings belonging to a same column ofthe memory sector 105 are connected in parallel to each other to a samebit line BL.

The memory cells 110 of a generic string are in particular connected inseries between a source select n-channel MOS transistor 115 s and adrain select n-channel MOS transistor 115 d. A generic, intermediatememory cell 110 in the memory cells string has the drain terminalconnected to the source terminal of an adjacent memory cell 110, and thesource terminal connected to the drain terminal of another adjacentmemory cell 110. One of the two end memory cells 110 located at the endsof the string has the drain terminal connected to the source terminal ofthe drain select transistor 115 d; the drain terminal of the drainselect transistor 115 d is connected to the corresponding bit line BL.Likewise, the other end memory cell 110 of the string has the sourceterminal connected to the drain terminal of the source select transistor115 s.

The gate terminals of the memory cells 110 in a same row of memory cellsof the sector are connected to a corresponding word line WL. The gateterminals of the drain select transistors 115 d belonging to a same rowof drain select transistors of the sector 105 are all connected to acorresponding drain select line DSL; similarly, the gate terminals ofthe source select transistors 115 s belonging to a same row of sourceselect transistors 115 s of the sector are all connected to acorresponding source select line SSL. The drain terminals of the drainselect transistors 115 d belonging to a same column of drain selecttransistors of the sector 105 are connected to a corresponding bit lineBL. Conversely, the source terminals of all the source selecttransistors 115 s in the sector 105 are connected to a common sourceline SL (which is typically kept at a reference voltage, or ground).

The memory cells 110 belonging to the same row are logically partitionedinto memory pages. Assuming, merely by way of example, that each sector105 includes 32K (i.e., 32,768) of bit lines BL, two memory pages of 2KByte each for each word line WL are defined: if the bit lines areassigned an index represented by an integer, a first memory pageconsists for example of the memory cells 110 belonging to the bit lineshaving an even index, and a second memory page consists of the memorycells 110 belonging to the bit lines having an odd index. It is pointedout that the number of memory pages that are associated with a genericword line may be higher than two, particularly multiples of two, such asfour, eight and so on; also, the size of the memory pages may bedifferent.

For accessing the memory cells 110, an address code ADD is provided tothe semiconductor device 100 through input/output pins ADD (in thepresent description, for the sake of simplicity, the input/output pinsand the corresponding inputs will be denoted with the same references)belonging to an input/output interface 125. In particular, theinput/output interface 125 includes all the components (for example, theabove-mentioned input/output pins and buffers), which are used forinterfacing the semiconductor device 100 with the outside environment.

The address code ADD is adapted to select a desired memory location (ina certain sector 105), for example for reading data or writing data fromor into the addressed memory locations.

Input/output pins I/O, also belonging to the input/output interface 125are provided for exchanging, with the outside environment, data DATAincluding data read from the flash memory during a reading operation,and data intended to be written thereinto; the exchanged data DATAcomprises for example the recovering data, which are used for restoringthe working environment of a computer.

The input/output pins I/O are also used for providing to thesemiconductor memory device 100 command codes C which are adapted tocontrol the operation of the semiconductor memory device 100, and moreparticularly for managing operating modes thereof. In the example atissue, the command codes C may comprise a command code HIBERNATION thatis used for activating the hibernation operation, and two furthercommand codes IDLE and SLEEP which are used for activating energy-savingoperation modes in which all the unnecessary components of thesemiconductor memory device 100 are shut down.

The address code ADD, the command codes C and the data DATA are fed to acommand interface 130, which latches them in response of a group ofactivating signals CTL. The activating signals CTL are provided from theoutside to the semiconductor memory device 100 through correspondinginput pins CTL. In detail, the activating signals CTL comprise anaddress code latch enable signal AL which activates the latching of theaddress code ADD by the command interface 130, and a command code latchenable signal CL which activates the latching of the command codes C,also by the command interface 130. Moreover, the activating signals CTLmay comprise other signals such as a write enable signal W, which whenasserted enables the latching of the data DATA during the writingoperation, and a read enable signal R, which, when asserted, controlsthe outputting of the data DATA during the reading operation.

The command interface 130 interfaces to a control logic unit 135 (suchas a finite-state machine), which is used for timing the operationsperformed by operation of the semiconductor memory device 100. Thecontrol logic unit 135 provides a set of control signals (denoted as awhole with Sc) used for driving the various components of thesemiconductor memory device 100. For example, the control signals Sccontrol the operation of a power managing circuit 140 which is adaptedto provide biasing voltages used for performing operations such as read,program, erase and verify operations on the semiconductor memory device100.

In particular, the power managing circuit 140 receives a supply voltageVdd, and outputs different biasing voltages Vhv; the voltages Vhv aregenerally higher in absolute value than the supply voltage Vdd, (suchas, 18V). For this purpose, the power managing circuit 140 includes acircuitry (for example, charge pumps and voltage level shifters) adaptedto generate the biasing voltages Vhv from the supply voltage Vdd.

In response to the command code IDLE, the control signals Sc activate anoperation mode referred to as IDLE MODE in which the charge pumps of thepower managing unit 140 are activated, and generate the voltage neededfor the generation of the high biasing voltages Vhv.

The latched address code ADD is supplied to an address decoder circuit150. The address decoder circuit 150, in response to the receivedaddress code ADD, generates a row selection code ADD_ROW and a columnselection code ADD_COL, used for selecting a memory page to beprogrammed during a program operation and to be read during a readingoperation.

The row selection code ADD_ROW and the column selection code ADD_COLconsist of respective string of bits.

In particular, the bits of the row selection code ADD_ROW are grouped intwo fields, wherein a first field is used to determine which sector 105has to be used during the operation of the semiconductor memory device100, whereas the remaining, second field is used for determining whichpage has to be programmed within the selected sector.

The row selection code ADD_ROW is supplied to a plurality 152 of rowselectors 155. In one embodiment of the invention, each row selector 155is associated with a respective memory sector 105, however, inalternative embodiments of the invention it may be provided that ageneric row selector is associated with two or more memory sectors.

In particular, each row selector 155, in response to the received rowselection code ADD_ROW indicating that the corresponding sector isselected, accesses to the sector 105 and selects the word line WL of thedesired page, as specified in the row selection code ADD_ROW, as well asthe drain select line DSL and the source select line SSL of thecorresponding strings.

The column selection code ADD_COL is supplied to a column selector 156.In one embodiment of the invention, each column selector 156 isassociated with a respective memory sector 105, however, in alternativeembodiments of the invention it may be provided that a generic columnselector 156 is associated with two or more memory sectors 105. Inparticular, the column selector 156 comprises a column decoder 157, apage buffer 158 and a bit line selector 159. The column decoder 157, inresponse to the column selection code ADD_COL, connects the input/outputpins I/O of the semiconductor memory device 100 to the proper sectionsof the page buffer 158. The page buffer 158 is exploited as a temporarystorage unit during read/write operations on the memory cells 110 of theselected page. For example, the page buffer 158 may include a relativelyhigh number of volatile storage elements (such as latch structures),adapted to properly biasing the bit lines of the memory sector dependingon the operation which has to be performed on the flash memory. The pagebuffer 158 has a storage capability equal to the storage capability ofthe generic memory page (in the example at issue, 2KBytes), and is thusadapted to manage the operations of reading the information stored inthe memory cells of a selected memory page, or writing new informationthereinto in one shot.

The memory page is selected by selecting a certain word line WL, and acertain packet of bit lines BL. The selection of the word line WL isperformed by the row selector 155, while the selection of the packet ofbit lines BL is performed by the bit line selector 159. Based on thecolumn selection code ADD_COL, the bit line selector 159 selects thedesired packet of bit lines BL and connects them to the page buffer 159.

In the example at issue, in which two memory pages of 2KBytes exist foreach word line WL, the packet of selected bit lines BL includes the16384 bit lines with even index, or the 16384 bit lines with odd index.

Assuming that the semiconductor memory device 100 has eight input/outputterminals I/O, when data DATA has to be written into the semiconductormemory 100, the page buffer 158 is replenished with the data DATAreceived serially in eight bits chunks (through the column decoder 157),and, after the page buffer 158 has been filled, the data DATA arewritten in parallel into the memory cells of a given, selected memorypage.

Similar considerations apply when the data DATA are retrieved from thesemiconductor memory device 100. The data read (in parallel) from thememory cells 110 of a selected page are temporarily stored in the pagebuffer 158. before being serially outputted in chunks of, for example,eight bits to the outside.

In operation, the semiconductor memory device 100 is put in anelectronic system, which for example includes, inter alia, a dataprocessor (not shown in the drawings). Just before a power down event,recovery data DATA are stored in the semiconductor memory device 100 sothat, when the power supply returns, the working environment before thepower down can be retrieved.

As discussed in the introduction of the present description, the timenormally required by the flash memory for programming data is ratherlong, and might not allow saving the recovering data when a power downoccurs. The reason of this resides in the fact that the programming ofdata is usually carried out in several steps, applying to the memorycells relatively light programming pulses to slightly change the memorycells' threshold voltage, and, after each pulse, verifying the cells'content.

According to an embodiment of the present invention, the semiconductormemory device 100 is adapted to implement a peculiar program operation,whereby the storage of the recovery data during a power down eventrequires a time significantly shorter compared to the normal programmingtime of the memory.

In particular, according to an embodiment of the present invention, thestorage of the recovery data in case of a power down is initiated bysending to the semiconductor memory device 100 the command codeHIBERNATION. Upon receiving this command code, the semiconductor memorydevice 100 understands that the data it will receive are recovery datathat are to be quickly saved before the power supply is cut off. Inresponse of the command code HIBERNATION the semiconductor memory device100 starts the page program operation. During the operation, therecovery data DATA are fed to the semiconductor memory device 100 andare progressively loaded into the page buffer 158, in chunks of, forexample, eight bits. The page buffer 158, through the bit line selector159, selects one packet of bit lines corresponding to the selected page.The recovery data are programmed page after page, until all therecovering data DATA are saved.

Every time a page of recovering data has been fully loaded into the pagebuffer 158, that page of recovery data DATA is stored into a page of thesemiconductor memory device 100.

In particular, considering two subsequent page program operations, thebit line selector and the row selector select respectively a packet ofbit lines and one word line. According to an embodiment of the presentinvention, in order to speed up the programming of the recovering data,the word lines that are selected for programming different pages ofrecovering data are coupled to different memory cell strings. In otherwords, for each string of memory cells a single memory cell 110 is usedfor storing the recovery data DATA, leaving the remaining memory cellsof the string in the erased state. The program operation of the memorycells of each string is performed by applying a relatively strongprogram pulse, adapted to guarantee that the threshold voltage of theselected memory cell in the string is brought to a value correspondingto the programmed state. No verify operation of the memory cell contentis needed during the programming, and this allows substantiallyshortening the programming time associated to each page programoperation, thereby the recovery data can be saved before the powersupply is cut off. Indeed, when all the memory cells of a string areused to store data, it is important that, when a generic memory cell ofthe string is programmed, its threshold voltage does not exceed thepassing voltage, otherwise when another memory cell of the string isaccessed for reading its content, the former memory cell is not properlyconductive. When, according to an embodiment of the present invention,only one memory cell in each string is used to store data, the remainingmemory cells in the string are not be used to store the recovery data,and are left in the erased state; these other memory cells will thus notbe accessed for reading their content; for this reason, it is notimportant that the threshold voltage of the memory cell used for storingthe recovering data does not exceed, when the memory cell is programmed,the reading pass voltage.

The memory page where the recovery data will be stored is selected byselecting (and properly biasing) a given word line WL, and a packet ofbit lines BL (for example, the bit lines with even index), which arethen connected to the page buffer 158.

For programming the selected memory cell belonging to the selectedstring, a significantly high programming voltage (such as 18V) isapplied to the memory cell control gate, while the control gates of theremaining memory cells of the string are driven by a programming passvoltage (such as, 10V), typically lower than the programming voltage,but sufficient to turn them on. The programming voltage and theprogramming pass voltage are provided by the power managing circuit 140.The bit line corresponding to the selected string is biased at a voltagethat depends on the data to be programmed into the considered memorycell: if the data to be programmed is a “0” (by convention,corresponding to a memory cell with a positive threshold voltage), thebit line is biased at the reference voltage (such as 0V); if instead thememory cell is intended to store a “1” (which, by convention,corresponds to an erased memory cell, with a negative threshold voltage;thus, when the data to be stored is a “1”, the corresponding memory cellis left in the original state), the bit line is biased at a highervoltage, such as the memory device supply voltage Vdd (for example, 3V).The other bit lines of the memory sector 105 (for example those havingodd index) are brought to the supply voltage Vdd. The control terminalof the drain select transistor 115 d corresponding to the selectedstring is driven by the supply voltage, thus in case the data to beprogrammed is a “0” the drain select transistor 115 d is turned on,whereas in case the memory cell is intended to store a “1” the drainselect transistor 115 d is kept off. The source select transistor 115 sreceives at its control terminal the reference voltage, and thus remainsturned off.

In detail, when a logic value equal to “0” has to be stored into theselected memory cell, the threshold voltage thereof has to be shiftedtowards higher values compared to the erased state (that corresponds toa logic “1”). For injecting electrons into the floating gate, asdescribed above the row selector 155 biases the word line WL to whichthe selected memory cell 110 belongs to the high programming voltage,(18V); the remaining word lines WL controlling the control gates of theother memory cells of the same string are all brought to the programmingpassing voltage (10V) sufficiently high to ensure that these memorycells are turned on irrespective of their programming state, butsufficiently lower than the programming voltage not to cause spuriousprogramming. All the other word lines WL (associated with differentmemory cells strings) are for example kept grounded. In such a way, theground voltage at the selected bit line, propagating through the drainselect transistor 115 d and the channel of the memory cells 110 in thestring, reaches the channel of the memory cell 110 to be programmed;here, the voltage difference of approximately 18V between the controlgate and the channel of the selected memory cell is sufficient to causeelectron tunnelling into the floating gate thereof.

When the data to be stored into the selected memory cell is a logicvalue equal to “1”, its threshold voltage has to remain unchanged (underthe assumption that the memory cell starts from an erased state). Forthis purpose, the voltage difference between the control gate and thechannel of the selected memory cell is kept lower than the programmingvoltage, so as to be insufficient for causing a charge injection intothe floating gate thereof. All the memory cells in the string are on,and, being the drain select transistor 115 d off, the voltage of thechannel of the selected memory cell is boosted toward relatively highvalues (such as, 7V) thanks to the capacitive coupling effect betweenthe selected memory cell and the remaining memory cells of the stringsuch that the voltage difference between the control gate and thechannel of the selected memory cell is not sufficient to causesignificant electron tunnelling, and the memory cell remains erased.

In this way, by applying to the selected bit lines the proper voltage,the cells 110 of the selected page to be brought to the logic value “0”are then programmed, while the other cells 110 are left unchanged (thatis in the erased state).

As mentioned above, according to an embodiment of the present inventionthe program operation is performed applying the high programmingvoltage, as a single programming pulse, which is suitable for bringingthe threshold voltage of the memory cells 110 of the selected pagetowards significantly high values. In such a way, the memory cellsbelonging to the selected memory page are certainly programmed afterhaving applied the single programming pulse, and no verify operation isneeded.

For example, it has been found that the time required for saving therecovering data from the page buffer 158 to the selected page rangesfrom 50 μs to 100 μs.

Using only one memory cell 110 for each string of the selected page forstoring the recovery data is also particularly advantageous forretrieving from the memory the recovery data DATA so as to restore theworking environment existing before the abrupt cut off of the supplyvoltage. For this purpose, a reading operation is performed forretrieving the recovery data DATA previously stored in the memory; inparticular, for accessing to the memory cells 110 that store therecovery data DATA, the row selector 155 brings all the word lines ofthe selected string to the reference voltage (0V). This is possiblebecause, according to an embodiment of the present invention, exceptionmade for the single memory cell used to store the recovery data DATA,all the other memory cells of the generic string corresponding to thepage to be read are always conductive (since they are deliberately leftin the erased state). Thus, during the reading operation, the stringsare conductive or not depending only on the state of the memory cell 110storing the recovery data DATA.

Moreover, performing the reading operation using the reference voltageas the reading voltage applied to the word lines may provide a strongreduction of the spurious hot carrier generation that normally causesthe soft programming of the unselected memory cells.

As a result, it is not necessary to adopt error-correcting-codetechniques for correcting the read data DATA and the performance of thesemiconductor device is improved.

A further advantage of the proposed solution consists of reduced powerdissipation thereof.

For example, simulations show that when supply voltage Vdd ranges from2V to 3.6V, during the IDLE mode the currents supplied to thesemiconductor memory device 100 are lower than hundreds of μA (such as100 μA), whereas during the HIBERNATION mode the currents supplied arelower than some mA (such as, 5 mA).

Referring to FIG. 2, a non-volatile semiconductor memory device 200according to another embodiment of the present invention is shown.

The semiconductor memory device 200 is integrated in a chip ofsemiconductor material, and includes one or more memory sector sets 205,each set 205 comprising a plurality of sectors 105. In the example atissue, each set 205 comprises five sectors 105, however the number ofsectors 105 that are associated with the generic set 205 may bedifferent (higher or lower) than five.

In particular, a column selector 210 is provided for each set 205. Thecolumn selector 210 comprises a column decoder 215, a page buffer 220and a bit line selector 225. Similarly to what described above, thecolumn decoder 215, in response to the column selection code ADD_COL,connects the input/output pins I/O of the semiconductor memory device100 to the proper sections of the page buffer 220 which is exploited asa temporary storage unit during read/write operations on the memorycells 110 of the selected page.

The semiconductor memory device 200 includes a temporary storage, e.g. abuffer, such as a RAM (acronym for Random Access Memory) 228. The RAM228 receives the data DATA from the command interface 130 and providesthem to the page buffers 220.

The page buffer 220 is adapted to perform a program operation whereinthe data DATA to be written into a selected memory page can be loadedinto the page buffer while another memory page is still being writtenwith data loaded in the page buffer at a previous time; in this way, thetime necessary for programming in sequence different memory pages isreduced. For implementing this operation, the page buffer 220 includestwo registers, namely a cache register 230 and a main register 235, eachone controlled by corresponding control signals Sc. In detail, the cacheregister 230 is exploited for loading, from the RAM 228, data to beprogrammed into the memory cells 110 belonging to the selected page. Themain register 235 is adapted to write into the selected memory cells 110the data DATA received from the cache register 230. This structurepermits programming a first memory page by means of the main register235 while the data to be written into a second memory page are loadedinto the cache register 230, thus speeding up the operation.

Similarly, when the data DATA have to be retrieved from the selectedsector 105, the read data DATA are firstly loaded in the main register235 and thereafter they are transferred into the cache register 230,which provides the data to the output, making them available to theoutside environment. The output of the data DATA from the cache register230 may occur simultaneously to the loading of further data DATA (fromthe sector 105) in the main register 235, so increasing the output datarate.

In particular, the cache register 230 and the main register 235 have astorage capability equal to the storage capability of the generic memorypage (in the example at issue, 2KBytes), and are thus adapted to managethe operations of reading the information stored in the memory cells ofa selected memory page, or writing new information thereinto. Inparticular, the recovering data DATA can be transferred from the mainregister 235 to the selected page in one shot.

The operation of the semiconductor memory device 200 will be describedreferring to the schematic flowchart depicted in FIG. 3, where the mainsteps of a method 300 are shown of performing a hibernation operationduring which the recovery data DATA are saved into the memory cells 110belonging to a set 205 of FIG. 2.

The storage of the recovery data in case of a power down is initiated bysending to the semiconductor memory device 200 the command codeHIBERNATION. Upon receiving this command code, the flash memoryunderstands that the data it will receive are recovering data DATA thatare to be quickly saved before the power supply is cut off. The recoverydata DATA (coming from the command interface 130) that are received bythe memory are sequentially sent to the RAM 228, and temporarily storedtherein (block 315). Thereafter, a loop is entered, wherein a firstportion of the recovery data DATA, which are stored in the RAM 228, istransferred into the cache register 230 (block 320). Then, the recoverydata DATA are moved from the cache register 230 to the main register 235(block 325). Afterwards, the recovery data DATA, which are in the mainregister 235, are written into the selected memory cells of the flashmemory 200, so as to guarantee the retention of the recovering data DATAalso during a power down event (block 330).

In particular, when a complete page of data has been loaded into themain register 235, the recovery data DATA are written into a selectedmemory page of the flash memory 200. In particular, considering twodifferent page program operations, the bit line selector and the rowselector select respectively a packet of bit lines and one word linecoupled to different memory cell strings. As in the previous embodiment,for each memory cells string a single memory cell 110 is used forstoring the recovering data DATA, leaving in the erased state theremaining memory cells of the string. Also in this case, the programoperation of the memory cells of each string is performed by applying arelatively strong programming pulse, adapted to guarantee that thethreshold voltage of the selected memory cell 110 is brought to thevalue corresponding to the programmed state. In such a way, no verifyoperation of the memory cell content is needed so shortening theprogramming time associated to each page program.

The above operations are iterated until all the recovering data DATAtemporally retained by of the RAM 228 have been written into the Flashmemory 200 (block 335).

In other words, when the amount of recovering data DATA exceeds thestorage capability of the cache register 230, i.e. one memory page, thedata are written in a number of iterations (which in particular dependson a ratio between a total number of bytes of the recovery DATA and thestorage capability of the cache register 230). One memory page of theselected sector 105 in the set 205 is used to store the recovery data,and the memory cells belonging to the other memory pages are left in theerased state. When a memory cell 110 of the string is already used forstoring the recovery data DATA, the string is not more utilizable forstoring further recovering data DATA.

In detail, a number of iterations N for writing the recovery data DATAinto the flash memory cells depends on the total number of bytes of therecovery data DATA to be stored, the storage capability of the RAM 228and of the page buffer 220.

In particular, the following relation holds:N=(TOTs/RAMs)*(RAMs/PBs)=TOTs/PBsin which the term TOTs indicates the total number of the bytes formingthe recovery data DATA to be stored, the term RAMs indicates the numberof bytes stored in the RAM 228, and the term PBs indicates the size inbytes of the registers 230 and 235.

The time required for storing the data DATA depends on the number ofiterations and more in particular on the predetermined number N. Inparticular, the time required by the semiconductor device 200 forperforming the hibernation operation follows the following relation:T=(N)*Tswhere in the term T indicates the total time required for storing therecovery data DATA into the flash memory 200, the term Ts indicates thetime required for transferring the data DATA from the main register 235to the selected sector 105 by using for the storing operation one onlymemory cell for each string.

For example, in case Ts=50 us and N=2, the saving of the recovery dataDATA requires approximately 100 μs.

It should be noted that the semiconductor device 200, having only onecolumn selector 210 for each set 205, allows reducing the semiconductorarea necessary for integrating the device, compared to solutions whereina column selector is provided for each sector 105.

It should be noted that the semiconductor memory devices 100 and 200according to the embodiments of the present invention can be implementedas stand-alone devices.

The fast write functionality according to the embodiments of theinvention herein described can be implemented in conventional NAND Flashmemory devices, making them adapted to be used as NVRAMs for performingthe fast data saving during the system power down.

Although applicable in general to any kind of electronic system, anembodiment of the present invention is in particular applicable toelectronic systems like that employed by the personal computers,particularly albeit not limitatively hard disk drives as schematicallyshown in FIG. 4. In particular, an exemplary scheme of an electronicsystem 400 including a hard disk drive 405 and a motherboard 410 limitedto some blocks relevant for the understanding of an embodiment of thepresent invention is shown.

The hard disk drive 405 is used for the long-term retention of the dataof a personal computer. The hard disk drive 405 comprises a disk controlunit 415 which interfaces to a ferro-magnetic storage support, such asglass platters covered by a ferromagnetic material and a plurality ofheads adapted for writing/reading the data (indicated in the whole as420) on the platters. According to an embodiment of the presentinvention, a NAND Flash memory featuring a fast write functionality asdescribed in the foregoing is used to form a flash write buffer 425 inthe hard disk drive 405, interfacing the control unit 415. The hard diskdrive 405 and the motherboard 410 communicate with each other throughcorresponding interface circuits (such as a Serial Advanced TechnologyAttachment interface) 430 and 435. Moreover, the motherboard 410includes all the components, such as a ROM memory 440, a RAM memory 445and a microprocessor 450 which are used for managing the operations,which are required for the personal computer.

In the example at issue, the flash write buffer 425 is used as a cachenon-volatile memory device. In other words, when data have to be storedinto the hard disk 405, the data are firstly quickly stored into theflash write buffer 425, and they are then are transferred to theferromagnetic support 420.

In such application, the proposed semiconductor devices are employed forreducing the power consumption and thus to guarantee better performanceof the electronic system in which they are embedded. Indeed, thanks tothe fact that the platters of the ferro-magnetic support 420 are off andthe heads are parked during the programming of the flash write buffer425, it is possible to reduce the total power consumption during theoperation of the hard disk drive 405. In particular, it has been foundthat by using the flash write buffer 425 the reduction of the powerconsumption is approximately of the order of 87% with respect to theconventional hard disk drive using only the ferro-magnetic support forpermanently storing the data.

Moreover, with the platters of the ferro-magnetic support off and theheads parked during the most of the time, the ferro-magnetic support isless likely to be damaged.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention.

What is claimed is:
 1. A memory device comprising: a data node; areference node; a group of non-volatile memory cells serially coupledbetween the data node and the reference node; and a control circuitoperable to store data in a selected non-volatile memory cell of thegroup of non-volatile memory cells and to perform no verification of astate of the selected non-volatile memory cell after the data is storedin the selected non-volatile memory cell.
 2. The memory device of claim1, wherein the control circuit is operable to store data in the selectednon-volatile memory cell using a single programming pulse.
 3. The memorydevice of claim 1, wherein the control circuit is operable to store nodata in other memory cells within the group of non-volatile memory cellsuntil after data from the selected non-volatile memory cell is read. 4.The memory device of claim 1, wherein the control circuit is operable toleave other memory cells within the group of non-volatile memory cellsin an erase until after data from the selected non-volatile memory cellis read.
 5. The memory device of claim 1, wherein the control circuit isoperable to store data in the selected non-volatile memory cell inanticipation of a circuit transitioning from a first operating mode to asecond operating mode.
 6. The memory device of claim 1, wherein thecontrol circuit is operable to store data in the selected non-volatilememory cell in a program operation in response to a command associatedwith a power down event.
 7. The memory device of claim 6, wherein thecontrol circuit is operable to perform the fast programming operation ina shorter time compared with a normal programming operation.
 8. Thememory device of claim 1, further comprising word lines, wherein each ofthe word lines is coupled to a respective non-volatile memory cell amongthe group of non-volatile memory cells, and the control circuit isoperable to apply a same voltage to the word lines during retrieving ofdata from the selected non-volatile memory cell after data is stored inthe selected non-volatile memory cell.
 9. The memory device of claim 8,wherein a value of the voltage is zero.
 10. The memory device of claim1, wherein the reference node comprises a source node.
 11. A methodcomprising: storing data in a selected non-volatile memory cell of agroup of non-volatile memory cells serially coupled between a data nodeand a source node of a flash memory device; and performing noverification of a state of the selected non-volatile memory cell afterthe data is stored in the selected non-volatile memory cell.
 12. Themethod of claim 11, wherein storing data in the selected non-volatilememory cell is performed after receiving data representing a transitionfrom a first operating mode to a second operating mode.
 13. The methodof claim 11, wherein storing data in the selected non-volatile memorycell includes applying a single programming pulse to a word line coupledto the selected non-volatile memory cell.
 14. The method of claim 11,further comprising: applying a same voltage to word lines duringretrieving of data from the selected non-volatile memory cell, whereineach of the word lines is coupled to a respective memory cell among thegroup of non-volatile memory cells.
 15. The method of claim 11, furthercomprising: applying zero volts to word lines during retrieving of datafrom the selected non-volatile memory cell, wherein each of the wordlines is coupled to a respective memory cell among the group ofnon-volatile memory cells.
 16. A memory system comprising: a firstcircuit die including a group of non-volatile memory cells seriallycoupled between the data node and a source, and a control circuitoperable to store data in a selected non-volatile memory cell of thegroup of non-volatile memory cells and to perform no verification of astate of the selected non-volatile memory cell after the data is storedin the selected non-volatile memory cell; and a second die including acontrol unit coupled to the first die.
 17. The memory system of claim16, wherein the first circuit die includes a flash memory device, andthe group of non-volatile memory cells is included in the flash memorydevice.
 18. The memory system of claim 17, wherein flash memory deviceis configured as a cache non-volatile memory device.
 19. The memorysystem of claim 16, wherein the control unit is coupled to an interfaceto receive communication from a microprocessor.
 20. The memory system ofclaim 19, wherein the interface includes a Serial Advanced TechnologyAttachment interface.